1. Statement of the Technical Field
This invention relates generally to a lightning discharge and more particularly to an improved method and apparatus for detecting, classifying, and locating lightning.
2. Description of the Related Art
A lightning discharge is a natural phenomenon that produces a non-cooperative electrical field waveform which varies drastically in intensity and duration. The lightning discharge, in its entirety, whether it strikes the ground or not, is usually termed a lightning flash or just a flash. A lightning discharge that involves an object on the ground or in the atmosphere is sometimes referred to as a lightning strike. The terms stroke or component stroke apply only to components of cloud to ground discharges. Each stroke involves a downward leader and an upward return stroke and may involve a relatively low level of continuous current that immediately follows the return stroke. Transient processes occurring in a lightning channel while it carries continuing current are termed M-components. First strokes are initiated by stepped leaders while subsequent strokes following previously formed channels are initiated by dart or dart stepped leaders.
The four types of lightning are: A) downward negative lightning; B) upward negative lightning; C) downward positive lightning; and D) upward positive lightning. Discharges of all four types can be viewed as effectively transporting cloud charge to the ground and, therefore, are usually termed cloud to ground (CG) discharges. It is believed that downward negative lightning flashes, type A), account for about 90 percent or more of global cloud to ground lightning, and that 10 percent or less of cloud to ground discharges are downward positive lightning flashes, type C). Upward lightning discharges, types B) and D), are thought to occur only from tall objects (higher than 100 meters) or from objects of moderate height located on mountain tops. The majority of lightning discharges do not involve the ground. These are termed cloud discharges and sometimes are referred to as IC's. Cloud discharges include intra-cloud, inter-cloud, and cloud to air discharges.
Downward negative lightning discharges are discharges that are initiated in a cloud. Initially they develop in an overall downward direction, transport a negative charge to ground, and probably account for about 90 percent of all cloud to ground discharges.
Positive flashes are defined as those transporting positive charge from cloud to earth. It is thought that less than 10 percent of global cloud to ground lightning is positive.
Upward lightning discharges are initiated by leaders that originate from stationary grounded objects, usually tall towers, and propagate upward toward charged clouds overhead. Upward lightning, as opposed to normal downward lightning, would not occur if the object were not present and, therefore, can be considered to be initiated by the object. Objects with heights ranging from approximately 100 to 500 meters experience both downward and upward flashes, where the number of upward flashes increases with the height of the object. Structures which are less than 100 meters or so are usually assumed to be struck only by downward lightning, and structures with heights greater than 500 meters or so are usually assumed to experience only upward flashes.
The term cloud discharge is used to denote three types of lightning: I) intra-cloud discharge, those occurring within the confines of a thundercloud; II) inter-cloud discharges, those occurring between thunderclouds; and III) air discharges, those occurring between a thundercloud and clear air. It is thought that the majority of cloud discharges are of the inter-cloud type, although no reliable statistical data are found in the literature to confirm that this is the case. Often the abbreviation IC (for intra-cloud) is used to refer to all cloud flashes. It is reported that intra-cloud and cloud to air lightning discharges produce similar overall electric field charges. Approximately three-quarters of lightning discharges do not contact the ground, although this fraction depends on the storm type, the stage of storm development, and possible other factors. The early stages of thunderstorm development tend to be dominated by cloud discharges. Ten of more cloud flashes may occur before the first cloud to ground flash occurs.
A variety of systems and methods have been developed for identifying the occurrence and location of lightning, such as cloud to ground lightning, intra-cloud lightning and Inter-cloud lightning. Currently, lightning discharges are detected using envelope detection or amplitude threshold techniques which are limited to detection above the noise level.
In prior art lightning detection systems, three or more sensors are spaced apart to remotely detect the electric and magnetic fields of lightning discharges. Such discharges may be between a cloud and the ground (CG) or within a cloud (IC). Information from the sensors is transmitted to a central location, where analysis of the sensor data is performed. Typically, at least the time of occurrence and location of the discharges are determined from data provided by the sensors.
Remote sensors of lightning detection and data acquisition systems typically detect electric and magnetic fields of both CG and IC lightning flashes, which are composed of many discharges. It is important to be able to distinguish between the two types of flashes. To that end, remote sensors often look at the low frequency (LF) and very low frequency (VLF) emissions from lightning discharges. The electrical signals produced by LF and VLF detectors are ordinarily integrated prior to analysis to produce a waveform representation of the electric or magnetic discharge field, as the antenna inherently responds to the time derivative of the field. Analyzing signals representative of either an electric or magnetic field to distinguish CG and IC discharges is referred to as performing waveform analysis. There are several criteria for distinguishing between CG and IC events. One well known method of distinguishing lighting signals both in the LF and in the VLF range is to examine the time that passes from a peak in a representative signal to the instant it crosses a zero amplitude reference point. This is referred to as a peak to zero (PTZ) method of analysis. A relatively short PTZ time is a good indication that an IC discharge has occurred.
Another well known method of distinguishing is referred to as a bipolar test wherein the representative signal is examined for a first peak and a subsequent peak of opposite polarity which is greater than a predetermined fraction of the first peak. Such an occurrence is another good indication of an IC discharge. Yet another test for IC discharges is the presence of subsequent peaks of the same polarity in a representative signal greater than the initial peak. This is predicated on the fact that some IC discharges have a number of small and fast leading electromagnetic pulses prior to a subsequent larger and slower pulse. In the absence of such criteria indicating that the discharge is an IC discharge, it is ordinarily assumed to be a CG discharge. Even with the application of all established criterion for distinguishing between CG and IC events, some events are still misclassified.
An alternate method of lightning detection is to monitor very high frequency (VHF) radiation from lightning discharges. However, VHF detection systems must be able to process information at extremely high data rates, as VHF pulse emissions in IC lightning occur approximately one tenth of a millisecond apart. Additionally, VHF systems can only detect lightning events that have direct line of sight to the sensor. One such system is currently in use by NASA at Kennedy Space Center in Florida. However, this system is further restricted to line of sight between the sensors and the central analyzer as it uses a real time microwave communication system.
Analog sensors operating at LF and VLF frequencies are difficult to tune for both CG and IC lightning discharges. The median amplitude of a CG field signal is about an order of magnitude greater than the median amplitude of an IC field signal. Optimizing the gain of one of these sensors to detect IC events often causes the sensor to become saturated with the much greater energy of nearby CG lightning discharges. Therefore, it is customary to adjust the gain to accommodate both types of field signals, reducing a sensor's ability to detect IC events. As distant IC lightning discharges become attenuated by propagation over the ground, they become difficult to distinguish from background environmental noise.
Once a lightning occurrence is detected, several different methods can be used for determining the location of the lightning strike. One such method employs a time difference of arrival (TDOA) discrimination scheme. Systems using this method typically include three or more monitoring stations which are geographically separated by some distance. Each station includes a lightning stroke detector and a timing signal generator synchronized with the timing signal generator at each other respective detection location. When a lightning stroke occurs, the time of detection of the ground stroke is recorded at each station. This timing information is communicated from each of the monitoring stations to a central processing station. By using timing data from each monitoring station, the location of the lighting stroke can easily be determined. For example, in the case where four monitoring stations are used, an algorithm for determining the location of the ground stroke is fairly simple, being reduced to the solution of a set of linear equations.
The state of the art of lightning detection and data acquisition systems is generally represented, in part, by several patents. Krider et al. U.S. Pat. Nos. 4,198,599 and 4,245,190 describe a network of gated wideband magnetic direction finding sensors. These sensors are sensitive to return strokes in CG lightning flashes. In U.S. Pat. No. 4,198,599, discrimination and classification is accomplished by examining the shape of the time-domain field waveform. A short rise time (time from threshold to peak) results in a representative signal being placed in an analog track and hold circuit while further analysis is performed. These sensors are designed with CG discharges being of primary interest. Any IC lightning discharges that are detected are discarded. However, both CG and IC events that meet the short rise time criteria and a simple test of event duration result in a significant amount of sensor dead-time.
Another patent of interest is Markson et al. U.S. Pat. No. 6,246,367 wherein a lightning detection system utilizes an analog-to-digital converter (“ADC”) to provide continuous processing of representative field signals. Markson describes using a bipolar comparator to distinguish between positive and negative polarity versions of a particular pulse that is inferred to be the first broadband radiation pulse in either a CG or an IC flash. Markson also uses a data correlation process and time-of-arrival difference location method. Markson explicitly uses a high pass filter to block most low frequency components of representative field signals, which are not necessarily useful for detecting the initial pulse in the flash. Limitations of the Markson patent are the specific use of the HF frequency range and detection and processing of only the first pulse in each flash.
Accordingly, there is a need for improved lightning detection and data acquisition systems in several respects. First, an improved signal conditioning method is needed. CG events are normally an order of magnitude larger than IC events at LF, due to the channel length and amount of current which flows during a CG return stroke. Increasing the gain, or equivalently reducing the event threshold, results in CG events saturating an analog detection and evaluation system or the pick-up of significant amounts of noise. Reducing the gain, or equivalently increasing the event threshold, results in inefficient detection that masks IC events. There is a need to reduce the effect of this magnitude difference between CG and IC signals while removing unwanted noise components. An interesting aspect of both electric field and magnetic field antennae is that they produce a signal which is proportional to the time derivative of the electromagnetic field they are detecting. These differentiating antennae actually reduce the magnitude disparity between IC and CG differential representative signals. However, current generation sensors invariably impose integration methods to convert the differentiated field signal to one representative of the electromagnetic field without making use of the fact that the antenna itself reduces dynamic range requirements. Additionally, there is a need for an improved classification method for distinguishing between lightning types.
Another need in the industry is the ability to program remote sensors with new or different waveform analysis techniques.
Thus, a need exists for a complete lightning detection system which can detect CG and IC events and determine their location, magnitude, and time of occurrence.